What Size Pool Heater Do I Need For My Pool?

This is a how-to guide to take you through the process of measuring your pool, spa or hot tub to work out what size pool heater you require. To work out the size of pool heater requires that you match your pool to the BTH/Hr rates specified by the pool manufacturer. To do this is fairly straight forward. Matching swimming pools require knowing the surface area and for spas and hot tubs the size in gallons. We have done this, using Hayward H Series Pool Heaters as an example.

Should You Oversize Your Pool Heater?

Hayward pool heaters are extremely efficient and the amount of energy lost is low. Because of this efficient energy transfer the more heat you can run through a unit the faster it will heat the water. Therefore, if you can afford to go for a larger unit then do so.

If possible we would always recommend going for the next model up – especially if you are borderline requiring a larger unit. This is even more backed up when you look at the difference in prices in the next sized unit – you will find it is not that much more.

How to Calculate Swimming Pool Surface Area

Use the following guides on how to work out the what size pool heater your need and then use the following table to identify the model (using Hayward H Series Pool Heaters as an example) that will be suitable for your requirements.

Pool Heater Model

Surface Area in Sq Feet

H150

450

H200

600

H250

750

H300

900

H350

1050

H400

1200

H500

1350

How to Measure Square, Rectangular or Oval Pools

This is the easiest type of pool to measure. You simply multiply the length by the width in feet.

For example:

A 36 foot long by 18-foot wide pool above ground pool

36 x 18 = 648 sq feet

So in this instance we would recommend purchasing the H250 or if you want to oversize your pool heater slightly for faster water heating the H300.

How to Measure Circular Pools

This again is pretty straight forward. Simply measure the radius of the pool and multiply it by itself and then by 3.14 (also known as pi to the eagle-eyed among you).
To get the radius measure the width of the pool through the centre and half it.

So for example:

Our pool is a 24-foot circular pool so the radius = 12 foot.

12 x 12 x 3.14 = 452.16 sq feet.

So here we would need an H200 or the H250 to oversize your heater.

How to Measure Odd Shaped Pools

This is the mot complex type of pool to measure so we often have to take a reasonable guess. The way to do this would be to take measurements width ways across the pool first with the narrowest and then with the widest.

Once you have the widest and narrowest measurements add them together and multiply this figure by the length. Then take this figure and multiply it by 0.45. this should give you a rough estimate so to be on the safe side we would always recommend oversizing your pool heater in this instance.

So for example our pool was:

(8 foot + 12 foot) x 24 foot x 0.45 = 216 sq feet.

Here we would recommend going for the H150 as there is plenty of wriggle room with this heater able to cope up to 450 sq feet of surface area.

How To Measure a Spa or Hot Tub

Measuring a spa or hot tub is slightly different to a swimming pool because we are more interest in volume than surface area. In addition to this the bigger the pool heater the faster it will heat the water as described in oversizing your all heater.

Once you work out the volume in gallons simply use the following table to identify the heater most suitable for you. If you already know the volume of your hot tub this should be even easier.

Taking the Measurements

First, we need to work out the surface area following exactly the same guide as for swimming pools.

Work out the average depth by taking a measurement at the deepest part and another at the shallowest and then divide this figure by 2.

Multiply the surface area x average depth x 7.5

For example, our hot tub is 12 feet across and is 3 feet deep in the middle and 2 feet deep at the edge of the top step inside the pool. So we perform the following calculation:

12 x (3+2/2) x 7.5

resolves to 12 x 2.5 x 7.5 = 225

So now we know out hot tub is 225 gallons. Using the sizing table and knowing I would like to have my hot tub ready to jump in within 10-15 minutes the H400 would be my choice of heater model.

Spa and Hot Tub Pool Heater Sizing Chart

The following table shows each of the Hayward H-Series Pool heaters and how long it would take in minutes to raise the water temperature by 30 degrees Fahrenheit given the size of the spa in Gallons.

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